1. Field of the Invention
The present invention relates generally to an improvement on a magnetic analyzer of oxygen, and, in particular, to a magnetic analyzer which can determine the concentration of oxygen in a gaseous sample from the variation of surface-pressure owing to the difference in magnetizing coefficients.
2. Description of the Prior Art
Magnetic analyzers of oxygen are classified roughly into those of the heat ray type and those which make use of variations in surface-pressure.
The former type is an analyzer which makes use of the cooling of heated rays and applies the principle that a magnetizing coefficient of oxygen, which is a paramagnetic gas, is reduced with a rise of temperature in accordance with Curie's law. That is to say, if heat rays arising from Joule heat or the like are located in an unequal magnetic field formed by a strong magnet in a measuring chamber made of non-magnetic materials, magnetic force is generated with a strength that is in proportion to the content of oxygen. Consequently, heat rays located in a magnetic field are cooled excessively compared with those not located in the magnetic field by virtue of such magnetic forces. On the basis of the above-mentioned principle, the concentration of oxygen in a gaseous sample can be determined by detecting the difference of temperature between these heat rays.
Accordingly, in the analyzers of this heat ray type, the measurement is remarkably influenced by the variation of thermal properties of coexistent gases. For example, the thermal conductivity and specific heat when hydrogen and carbon dioxide are present are remarkably different from oxygen alone. Such coexistent gases can be present in great quantities, and their contents are varied at times. Moreover, the analyzers of this type have a defect in that some special safety countermeasures are required against explosion in a case where the concentration of oxygen in explosive gases is measured because the temperature of heat rays is comparatively high.
The latter type of analyzer (i.e. the type which makes use of the variation in surface-pressure) is superior to the heat ray type in the above-mentioned points, and a high practical value can be expected for it.
The analyzers of the surface-pressure type apply the following principle. As shown in FIG. 1, a minute quantity of non-magnetic gas (for example, pure nitrogen), or a mixture of paramagnetic gases and non-magnetic gases (for example, air), is introduced into a passageway "a" of gases to be measured as a suitable comparison gas through a minute hole 6. This hole 6 is provided on one magnetic pole N in the portion where the magnetic poles N and S are arranged in said passageway "a" of gases to come closest each other so that the strongest magnetic field is formed.
In such a case, it has been known that kinetic pressure P, expressed by the following Quinke's equation, is generated in a vertical direction relative to boundary surfaces "b" and "c" between the comparison gas and the gas to be measured if the gases at said boundary surfaces "b" and "c" have magnetizing coefficients of x.sub.1 and x.sub.2, which respectively exist in said magnetic field having the strength of H: EQU P=K(x.sub.1 -x.sub.2)H.sup.2 ( 1)
where K is a constant including elements such as temperature; x.sub.2 is the magnetizing coefficient of the comparison gas; and x.sub.1 is the magnetizing coefficient of the gas to be measured. In a case when the condition of x.sub.1 &gt;x.sub.2 is satisfied, the surface-pressure is generated on the boundary surface between x.sub.1 and x.sub.2 in the direction from x.sub.1 to x.sub.2.
Accordingly, if the strength of said magnetic field H and any one of said magnetizing coefficients x.sub.1 and x.sub.2 are held constant, the magnetizing coefficient of another gas can be determined from the variation of said surface-pressure P. Then the content of a paramagnetic gas, such as oxygen, in the gas to be measured can be determined from the magnetizing coefficient.
However, all of the conventional analyzers of the surface-pressure type consist of one pair of magnetic pole pieces which face each other in the measuring chamber at a minute distance and one pair of false pole pieces made of non-magnetic materials having the same shape and size as said magnetic pole pieces. In such analyzers the variation of surface-pressure owing to the difference in magnetizing coefficients has been detected from the pressure inside a passageway of the comparison gas provided on one side of said magnetic pole pieces and the pressure inside a passageway of the comparison gas provided on one side of said false pole pieces which corresponds to said magnetic pole piece mentioned above. Consequently, the variation of surface-pressure owing to the difference in magnetizing coefficients is remarkably small, for example only 2 to 3.times.10.sup.-1 microbar per 1% O.sub.2. As a result, the measurement by means of the conventional techniques is quite difficult, and often led to error.
This is particularly the case in an analyzer in which the variation of surface-pressure is detected by a condenser-microphone. In such a case, the S/N ratio of the signal introduced in the oxygen content indicating portion is large because the variation of surface-pressure is minute. Also, although the voltage to be loaded on said condenser-microphone should be high in order to transform this minute variation of surface-pressure directly into an electrical output with high accuracy and high speed, it is known that the maximum possible voltage which can be loaded on said condenser-microphone is dependent upon the distance between a fixed pole and a condenser-film which serves as a movable pole, and upon the tension of said condenser-film. If a voltage higher than said maximum allowable voltage is loaded, the condenser-film is bent. Consequently, the distance between poles is shortened to start an electric discharge. In other words, Coulomb force hinders the possibility of increasing the voltage which can be loaded.